The anatomy of the female breast changes over age. During the reproductive years, the breast consists mainly of ductal, glandular, and fat tissue. This is interspersed with fibrous tissue providing support and attachment to the chest wall. Glandular and fibrous tissue are jointly called the fibro-glandular tissue. The breast glandular tissue is called the breast parenchyma and consists of 20 . . . 25 lobules (glands), responsible for milk production and are drained towards the nipple by numerous tiny tubes (ducts) that come together to form bigger ducts. Each milk-producing lobule contains a cluster or ring of cells. The sections of lobules and ducts are surrounded by fat for protection, and supported by the fibrous tissue. With age, ductal and glandular elements undergo atrophic changes and are increasingly replaced by fatty tissue. The breasts are held in place by ligaments that attach the breast tissue to the muscles of the chest. Breasts are covered by ordinary skin everywhere except the nipple and the aureoles around it.
The fibrous, ductal and glandular tissues appear as dark or “dense” on the X-ray mammogram. Fat, on the other hand, has a transparent, or lucent, appearance. The terms mammographic density (MD) and mammographic pattern are widely used to describe the proportion of dense/lucent areas in the breast presented on the mammogram. In the past, different methods of classification of mammographic parenchymal patterns have been proposed such as the Nottingham classification (5 patterns such as Normal (N), Glandular (G2, G1, G0), Dysplasia (DS-slight, DM-moderate, DY severe), prominent ducts (P1, P2) and indeterminate (IND)), Wolfe classification (4 categories) and Tabar-Dean classification (5 patterns).
Two major categories of breast cancer are lobular and ductal carcinoma.
Lobular carcinoma in situ (LCIS) is a condition of sharp increase of the number, appearance, and abnormal behavior of cells contained in the milk-producing lobules of the breast. The term “in situ” refers to an early stage of cancer and is used to indicate that abnormal cancer cells are present but have not spread past the boundaries of tissues where they initially developed. Though LCIS is not considered a cancer, women who are diagnosed with LCIS (also called lobular neoplasia) are at a higher risk of developing breast cancer later in life.
Ductal carcinoma in situ (DSIC) is the most common condition of early cancer development in the breast. Again “in situ” describes a cancer that has not moved out of the area of the body where it originally developed. With DCIS, the cancer cells are confined to milk ducts in the breast and have not spread into the fatty breast tissue or to any other part of the body (such as the lymph nodes). DCIS may appear on a mammogram as tiny specks of calcium (called micro-calcifications).
Both LCIS and DCIS may develop into invasive cancers (infiltrating lobular carcinoma or infiltrating ductal carcinoma) where cancer spreads into the fatty breast tissue or to any other part of the body (such as the lymph nodes), called metastases.
Mammography has become by far the most used and the most successful tool in the detection of early symptoms of breast cancer, which can often be signalled by the presence of micro-calcifications or masses. However, visual analysis—as performed by radiologists—remains a very complex task and many Computer-Aided Detection/Diagnosis (CAD) systems have been developed that support their detection and classification. Indeed, the impact of a CAD system on the detection efficiency of an experienced and respectively non-experienced radiologist has been investigated e.g. in (C. Balleyguier, K. Kinkel, J. Fermanian, S. Malan, G, Djen, P. Taourel, O. Helenon, Computer-aided detection (cad) in mammography: Does it help the junior or the senior radiologist?, European Journal of Radiology 54 (2005) (2005) 90-96. In both cases the CAD system has proven an effective support tool for the detection, though, its autonomy remains in doubt. Therefore, due to the complexity of the problem, automatic or semi-automatic systems still play only the role of a signalling tool for the radiologist.
In the CAD environment, one of the roles of image processing would be to detect the Regions of Interest (ROI) that need further processing for a given screening or diagnostic application. Once the ROIs have been detected, the subsequent tasks would relate to the characterization of the regions and their classification into one of several categories.
A classification system for mammographic lesions is offered by the American College of Radiology and is known as the B-RADS (Breast Imaging Reporting and Data System); apart from X-ray mammography, sections are included on ultrasound and magnetic resonance imaging (MRI) of the breast. The features that describe mammographic findings are illustrated by a line drawing depicting the feature, followed by several mammographic examples.
Examples of ROIs in mammograms are (a) calcifications, (b) tumors and masses, (c) the pectoral muscle, (d) the breast outline or skin-air boundary. Segmentation is the process that divides the image into its constituent parts, objects or ROIs. Segmentation is an essential step before the detection, description, recognition or classification of an image or its constituent parts, i.e. mammographic lesions, can take place.
A radiation image such as a mammogram typically consists of three main areas:
The diagnostic area comprises pixels corresponding to patient anatomy i.e. the breast. In general, the outline of this imaged area may take any shape.
The direct exposure area is the image region that has received un-attenuated radiation. Although this region has constant intensity corrupted by noise only, in-homogeneities in incident energy (e.g. X-ray source Heel effect) and receptor (e.g. varying storage phosphor sensitivity in computed radiograph may distort this pattern. In European patent application 1 256 907 a method is disclosed to estimate these global in-homogeneities retrospectively from the diagnostic image and flatten the response in all image parts in accordance with an extrapolated background signal.
The collimated areas—if any—appear on the image as highly attenuated pixels. The shape of these areas typically is rectilinear, but circular or curved collimation shapes may be applied as well.
Between these main areas in a radiation image, three different area transition types may be considered: diagnostic/direct exposure, diagnostic/collimated area, and direct exposure/collimated area boundaries.
Segmentation algorithms aim at detecting and separating of the set of pixels that constitute the object(s) under analysis. These techniques may be broadly classified according to the type of processing applied to the image. Region-based algorithms group pixels in the image according to suitable similarity criteria. In European patent application EP887 769 a region-based algorithm is disclosed to segment direct exposure areas by grouping pixels according to centroid clustering of the grey value histogram. Edge-based algorithms separate image pixels in high contrast regions in the image according to grey value differences of neighboring regions. In European patent application 610 605 and European patent application 742 536 an edge-based algorithm is disclosed to detect and delineate the boundaries between collimated areas and diagnostic areas on a single or multiply exposed image. Either in region-based and edge-based approaches, models may be used to restrict the appearance or shape of the segmented image areas to obey predefined photometric or geometric constraints. Examples of this paradigm are the so-called Active Appearance and Active Shape Models (AAM and ASM).